The present invention mainly relates to a radio frequency filter used for mobile communication equipment such as a mobile phone.
Recently, with the increase in needs for mobile communications equipment such as a mobile phone, the frequency band used for them has become insufficient in band width, giving rise to a trend of shifting to a higher frequency band. Accordingly, it is necessary for mobile communication equipment such as mobile phones to be compatible with a higher frequency. Above all, radio frequency filters used for them are becoming less in relative band width, and their component circuit elements are required to ensure higher dimensional accuracy.
A conventional radio frequency filter generally has a structure as shown in FIG. 5. The structure is as follows:
(1) First ground electrode 502 is formed on the top surface of first dielectric layer 501, and second dielectric layer 503 is laminated on ground electrode 502.
(2) Two resonator electrodes 504a and 504b are formed on the top surface of dielectric layer 503, and third dielectric layer 505 is laminated on resonators 504a and 504b. 
(3) First transmission electrode 506a, second transmission electrode 506b, third transmission electrode 506c, first capacitor electrode 507a and second capacitor electrode 507b are formed on the top surface of dielectric layer 505, and fourth dielectric layer 508 is laminated on these electrodes.
(4) Second ground electrode 509 is formed on the top surface of dielectric layer 508, and fifth dielectric layer 510 is laminated on ground electrode 509.
(5) Terminal electrodes 511a, 511b, 511c, 511d are formed on the side surface of dielectric layer 510.
Here, resonator electrode 504a and capacitor electrode 507a, and resonator electrode 504b and capacitor electrode 507b are respectively disposed so as to be at least partly opposed to each other via dielectric layer 505. As shown in FIG. 5, transmission electrode 506a, capacitor electrode 507a, transmission electrode 506b, capacitor electrode 507b and transmission electrode 506c are formed in order between terminal electrodes 511c and 511d, thereby forming a transmission line. In addition, terminal electrode 511a is connected to ground electrodes 502 and 509 for the purpose of grounding. Further, terminal electrode 511b is connected to ground electrodes 502 and 509, and to each end of resonator electrodes 504a and 504b for the purpose of grounding.
In a conventional radio frequency filter having a structure as described above, resonator electrodes 504a and 504b are of triplate structure sandwiched between two ground electrodes 502 and 509. Resonator electrodes 504a and 504b, one end of each electrode being grounded, operate as a quarter-wavelength resonator, that is, as a serial resonator. Moreover, these resonators are partly opposed to capacitors 507a and 507b respectively via dielectric layer 505, and operate as a parallel-plate capacitor. More specifically, two serial resonators with one end grounded are connected in shunt to the transmission line between terminal electrodes 511c and 511d via the parallel-plate capacitor. Accordingly, this filter operates as a band-stop filter having terminal electrodes 511c and 511d as input terminal and output terminal respectively.
In a conventional radio frequency filter having the structure and operation as described above, each electrode is formed by a plurality of electrode layers having predetermined dimensions inside dielectric layers. After the electrode layer forming process followed by a dielectric layer sintering process, a filter having a predetermined frequency characteristic is selected by measurement. However, as recent equipment becomes higher in frequency, causing the relative bandwidth of frequency characteristic of the filter to become narrower, electrode layers are required to ensure higher dimensional accuracy. Thus it is difficult for such conventional forming process to ensure the required characteristics. Moreover, since electrode layers have been formed by using screen printing technique in most cases, there has been a problem of worsening of the dimensional accuracy such as xe2x80x9cblurring of printing.xe2x80x9d Accordingly, the conventional method is unable to reduce the electrode dimensions while maintaining high accuracy and is limited with respect to reduction in size of the filter.
The object of the present invention is to provide a small-sized radio frequency filter with which the desired frequency characteristic can be obtained with simple configuration even in higher frequency band.
The radio frequency filter of the present invention includes an inductor component, capacitor component, and resistor component, and has a configuration as follows.
A conductor pattern is formed on the surface of a dielectric layer, forming at least one of the inductor component, capacitance component and resistor component.
An adjusting layer is formed so as to cover at least a part of the conductor pattern and serves to adjust the frequency characteristic of the filter.
The conductor pattern is usually formed on the surface of dielectric layer, and sintered thereafter.
The present invention has the following features.
(1) Forming an adjusting layer with a dielectric material, it is possible to increase the frequency adjusting range by changing the effective dielectric constant.
(2) Forming an adjusting layer with a magnetic material, it is possible to increase the frequency adjusting range by changing the effective permeability.
(3) Forming a conductor on the top surface of adjusting layer formed of the dielectric material or magnetic material, it is possible to enhance the change of the effective dielectric constant or effective permeability.
(4) Grounding the adjusting layer in (3), it is possible to further enhance the change of the effective dielectric constant or effective permeability.
(5) Forming a dielectric layer between the conductor pattern and the adjusting layer, it is possible to prevent the breakdown or damage of electrodes not to be adjusted.
(6) The adjustment can be regulated by changing the area of the adjusting layer for each conductor pattern of the adjusting object.
(7) The frequency can be adjusted higher by removing a part of the adjusting layer.
(8) The frequency can be adjusted lower by adding a part of the adjusting layer thereon.
(9) The capacity value of the capacitor can be adjusted by forming at least a pair of interdigital electrodes in the conductor pattern.
(10) The inductance value of the inductor can be adjusted by forming the conductor pattern, making at least one of the line width and the line space thereof not more than 60 xcexcm.
(11) Using intaglio-printing technique for forming the conductor pattern, it is possible to make the filter small-sized and adjustable.
(12) Using thin-film forming technique for forming the conductor pattern, it is possible to make the filter small-sized and adjustable.
The present invention having a configuration as described above may provide a small-sized radio frequency filter by which the desired frequency characteristic can be easily obtained with simple configuration even in a higher frequency band.